Separating protoactinium with manganese dioxide



Unite Patented Apr. 22, 1958 SEPARATING PROTOACTINIUM WITH MANGANESEDIOXIDE Glenn T. Seaborg, Chicago, 111., John W. Gofman, Berkeley,Calif., and Raymond W. Stoughton, Oak Ridge,

Tenn, assignors to the United States of America as represented by theUnited States Atomic Energy Commission No Drawing. Application November3, 1944 Serial No. 561,837

8 Claims. (Cl. 23-145) The invention relates to the preparation ofmasses and compositions of the isotope of uranium having a mass numberof 233, said isotope being designated as U or 23s An object of theinvention is to provide an improved method for preparing and isolatingP21 and U Other objects and advantages of the invention will becomeapparent as the following detailed description progresses.

In this specification and the claimsthe name of the element is used todesignate the element generically,

ill

of Pa and further that U so produced undergoes fission with neutrons ofsuch low energies as below 1 million electron volts (1 m. e. v.) andeven with thermal the preponderant component of the composition and preferably as a composition containing not in excess of about 10 percent byweight of impurities. Further we have discovered that U may beeffectively prepared as a concentrate by separating Pa from neutronirradiated thorium preferably by means of a carrier such as manganesedioxide and thereafter permitting the separated Pa to decay to U Inaddition we have found that efiective concentrates of these materialsmay be secured by treatment of neutron irradiated thorium which has beensubjected to high intensity irradiation forfa time suflicient to producePa -i-U in concentrations within a predetermined range.

The reaction of thorium with slow and moderately fast neutrons may besummarized as follows:

23.5 min 27.4 days 'p zaa 1 ,2513 p- Um 5- half life half life Thefission products which are. produced as a result of the fission of Uwith slow and-moderately fast neutrons are, -so far as we have been ableto determine, the same as those produced by the fission of U Theyconsist of a large number of elements which generally fall into a lightgroup with atomic numbers from to 46 incl.

and a heavy group with atomic numbers from 51 to iucl., and whichundergo beta decay. The fission products which have a half life of morethan three days will remain in the reaction mass in substantialquantities at least one month after the termination of the reaction, andthe removal or elimination of these products by our process isparticularly advantageous. Among these prodnets are: Sr, 1, Zr, Cb, Ru,Te, 1, Xe, Cs, Ba, La, and Ce of a 20 day half life, and Ce of a 200 dayhalf life.

In accordance with one embodiment of this invention,

the mass of thorium is subjected to the-action of neutrons, the majorityof which have energies below 1 million electron volts, and the reactionof the neutrons with the thorium is terminated prior to the time whenthe neutrons are absorbed by the U at the same rate that they areabsorbed by the Th This limit is approximately when the weight ratio ofU to unreacted Th is 1 to 100. In other Words, the reaction of Th withneutrons should preferably be terminated prior to when the amount of Uis approximately 1 percent of the amount of thorium present in the mass.When the reaction is terminated at or prior to this point there is alsono danger during the reaction of a substantial decomposition of the Utaking place by a nuclear self-sustaining chain a reaction.

Th in order to reduce the amount of fission products and at the sametime have a practical amount of U and Pa for isolation by batch process,the reaction is erminated at a weight ratio of U +Pa to 'l'h of not lessthan about 1 to 1 million and frequently between about 1 to 10,000, and1 to 1,000.

The reaction of thorium either in metallic state or a compound such asan oxide or carbonate of thorium with neutrons to produce Pa and U maybe carried out with neutrons from any suitable neutron source. Where theneutron source provides fast neutrons, the fast neutrons are slowed toneutrons having energies of below 1 millionelectron volts by interposingneutron slowing material between the fast neutrons and the thorium. Suchneutron slowing materials include carbon-containing,deuterium-containing, or hydrogen-containing material such as graphite,paratfin, or deuterium oxide. Suflicient neutron slowing material isused so that at least a majority of the neutronsare slowed to energiesof below about 1 million electron volts, since at higher energies thereis very little production of U and considerable fission of the thorium.We may interpose the neutron slowing material between the fast neutronsand the thorium-containing mass, or we may admix neutron slowingmaterial with the'thorium. An intimate mixture of thorium with neutronslowing material maybe readily obtained by using hydrated thoriumcompounds such as the compound Th(OH) .xH O. Since the slow neutronabsorption cross sectionofthorium is some ten to forty times larger thanthat of hydrogen, we may suitably use a ratio as high as about two tofour hydrogen atoms per thorium atom without losing any more than 10percent of the neutrons as a resultof absorption by hydrogen.

While neutrons obtained from any suitable source of high neutronoutputmay be used, it is desirable ;to subject the thorium to neutronsfrom a high intensity source in order that suitable concentrations of Paand U may be obtained in a reasonable length of time.

Preferably the thorium is subjected to slow neutrons from a source ofneutrons capable of supplying at least 5 neutrons per second to thethorium mass and where a relatively high concentration of U +Pa isdesired shouldweigh no more than about 20 tons. Preferably this massshould be of such a thickness that at least percent and preferably 75percent or more of the neutrons supplied are absorbed. Such high neutronintensity may be obtained by subjecting thorium to the action ofneutrons obtained by slowing down secondary neutrons obtained from aself-sustaining chain reaction of U U or 94 with neutrons.

By placing thorium adjacent to a neutron chain reacting mass comprisinguranium and/or 94 in amount suflicient to establish a self-sustainingneutron chain reactiondispersed in a neutron slowing medium such ascarbon or D 0, between.5 10 and 10 neutrons per second are supplied tothe thorium and at least 50 to 75 percent are absorbed so that a ratioof U +l a to Th of more than 1 to 1 million may be attained in areasonable length of time, such as one to three months. In such a casethe degree of bombardment desired may be completed before thepreponderant amount of Pa formed has decayed to U The method of chemicalseparation here involved is based upon the initial separation andrecovery of Pa folowed by conversion of the Pa to U and the subsequentseparation of these two elements. 7

A suitable method which is particularly effective for isolatingla and Udescribed below is given by way of example and it is to be understoodthat the invention is not limited to the details. The details of thismethod for isolating Pa and U were developed by Working with a number ofsmall samples of these radioactivities formed in small bombardments, andwith part of the isotopes which were obtained from 5 kg. of thoriumnitrate .(Th(NO .4H O) which was bombarded in the form of an aqueoussaturated solution with neutrons obtained from about 14,000micro-ampere-hours of deuterons on beryllium. The separations werecarried out before any appreciable quantity of Pa had decayed to UExample 1 TheSkg. of neutron bombarded thorium nitrate containing Pa +Uwas diluted up to 26 liters with water and'then made-approximately 0.5 Nin nitric acid. To this total solution was added about 400 grams ofmanganous chloride. The solution was heated and the manganese wasprecipitated as manganese dioxide from the hot solution by the additionof a soluble manganate or permanganate, especially an alkali manganateor permanganate such as potassium permanganate. Each precipitate ofmanganese dioxide was centrifuged out separately. The protoactinium iscarried down with the manganese dioxide in a substantially quantitativemanner under these conditions.

The combined manganese dioxide precipitates were dissolved in a mixtureof hydrogen peroxide and hydrochloric acid. After decomposing thehydrogen peroxide by boiling, about 200 mg. of zirconium oxychloridewere added to this solution, the zirconium was precipitated as zirconiumphosphate by the addition of phosphoric acid, and the precipitate wascentrifugedout separately. Zirconium phosphate carries down theprotoactinium essentially quantitatively, thereby separating the latterfrom the relatively large amount of manganese dioxide with which it wasoriginally associated.

It was now necessary to remove the Pa from the rather large amount ofzirconium with which it was present, and to purify it from uraniumandthorium. The zirconium phosphate containing protoactinium phosphate and.other phosphates was brought into solution by treatment. with. dilutehydrofluoric acid. This step also gave a separationfrom thorium, whichwas converted to the insoluble fluoride at this point and sep- I point.

arated. The solution was cooled with ice-water and zirconium andprotoactinium hydroxides were precipitated by adding dilute sodiumhydroxide solution; the solution must be kept cold in this precipitationsince otherwise difilculty will be experienced in redissolving theprecipitated hydroxides. The hydroxide precipitate was then dissolved innitric acid and the protoactinium was carried away from most of thezirconium by another series of manganese dioxide precipitations. Some ofthe zirconium comes along with the manganese dioxide precipitate in thisprocedure so that a-further removal of the zirconium, as well as themanganese, from the protoactinium was necessary. This involved goingthrough the above described cycle two more times, finally ending up witha small zirconium phosphate precipitate containing substantially all theprotoactinium that was originally present.

This zirconium phosphate precipitate, which now contained only about 10mg. of zirconium, was dissolved in hydrofluoric acid, and the zirconiumand protoactinium then precipitated as the hydroxides as describedabove. This hyrdoxi-de precipitate was then converted to thecorresponding nitrates by dissolving in nitric acid. Anotherprecipitation of the zirconium phosphate (carrying the protoactinium).was then made in order to be sure that all the uranium and thorium, toamounts less than a microgram, were removed. The final Zirconiumphosphate was converted into zirconium sulphate by dissolving inhydrofluoric acid, precipitating the zirconium as the hydroxide,dissolving the hydroxide in sulphuric acid, and evaporating to dryness.The protoactinium accompanies the zirconium through this series ofconversions. The final zirconium sulphate (containing about 10 mg. ofzirconium and. the protoactinium) was dissolved in about 30 cc. of 0.33M solution of ammonium fluoride or other alkali fluoride such as KP andthe hydrogen ion concentration of the solution was adjusted until itcorresponded to that of the methyl red indicator end The Pa was nowisolated by an electrolytic procedure; in this procedure the Pa wasdeposited in a very thin adherent layer and was separated from thezirconium at the same time. The electrolysis chamber consisted of aclear Bakelite chamber fitted with a brass plug which screwed into thelower end and upon which rested a copper disk upon which the Pa wasdeposited. This copper disk served as the cathode, while a motordrivenplatinum stirrer served as the anode. The electroplating was carried outfor about 8 hours at about 100 milliamperes of current, applying about15 volts across the electrolysis chamber. The Pa is plated out nearlyquantitatively (about 90 percent) under these conditions. No zirconiumplates out under these conditions, so that a good separation ofprotoactinium from zirconium is obtained by this procedure. The Pa maythen be stored until it has decayed substantially to U or it may beallowed to decay partially to U and the U separated from the Pa by anymethod of separating uranium from protoactinium.

By way of example, a separation of U from Pa was made by us as follows:

The thin plated film containing the Pa and U and which had aged abouttwo months was dissolved in 6 N hydrochloric acid to which a few dropsof 6 N nitric acid had beentadded. Since asmall amount of copper wasdissolved from the backing plate in this procedure this was removed byan H 8 precipitation from the acid solution. No Fa on U was carried downby the copper sulphide in this precipitation. After the removal of the HS and the reduction of the amount of 6 N hydrochloric acid to about 200., by boiling the solution, there was added about 0.1 mg. of zirconiumas zirconium oxychloride. There was then added about a hundredth of acc. of percent phosphoric acid and the precipitated zirconium phosphatewas removed by centrifugation.

' protoactinium.

The major portion of the Pa approximately 70 percent or more, wasremoved in this step. The yield of Pa in this precipitate is lower thanthat obtained when a large excess of phosphoric acid is used; however,an excess of phosphoric acid must be avoided because the acidity of thesolution must be kept very low in the electrolysis procedure whichfollows: It is essential that less than 0.1 percent of the Pa remainwith the U to avoid the simultaneous electrolytic deposition of Pa withthe U and hence further separations were necessary. Further separationsof the Pa from the U were effected by adding zirconium, about 0.1 mg. ata time in successive portions, to the solution. The Pa was not carrieddown quantitatively by the zirconium phosphate under these conditions sothat it was necessary to make about 15 precipitations of zirconiumphosphate, by the addition of zirconium to the phosphoric acid solutionin this manner. After performing this number of precipitations, lessthan. 0.1 percent of the Pa remained in the solution. Practically all ofthe U remains in solution in this procedure; however, it should beemphasized that in order for this to be the case it is necessary thatthe Pa? be removed with small successive portions of zirconium phosphatein the manner which has been described. 7

The volume of the solution was then reduced, by evaporation, to a volumeof about 0.1 cc. To this solution, which contained about 0.10 cc. of 6 Nhydrochloric acid and of the order of 0.001 cc. of 85 percent phosphoricacid, there was added 0.02 cc. of glacial acetic acid and the solutionwas diluted by theaddition of cc. of water. This solution was placed inan electrolysis chamber which consisted of a clear Bakelite chamberfitted with a brass plug that screwed into the lower end and upon whichrested a platinum foil that the U was to be deposited upon. Theelectroplating was carried out for about 8 hours at about 90milliamperes of current applying about 8 volts across the electrolysischamber (the actual electrode potential'for U 3 is not known). The U isplated out nearly quantitatively (greaterrthan 90 percent) under theseconditions and the plate usually appears 'as a well adhering film of.pure U (probably in the form of the trit aoctaoxide,,U O together with asmall amount of, organic matter which can be burned off theplate. Itshould be emphasized that in order for the uranium to be quantitativelydeposited the acidity of the solution must be kept very low as describedabove. (In addition, it may be pointed out that this electrolysis may beperformed from an 0.3 M ammonium fluoride solution, with approximatelythe same yields, and in some experiments we have used this procedure.)The weight of the U sample prepared in the manner outlined above, asdetermined from its alpha activity and from the halflife of U (1.2 10years), was found to be 3.8 micrograms.

The preferred method for separating protoactinium from zirconium is theelectrolysis from ammonium fluoride solution, described above, whereinmicrogram (or less) quantities of protoactinium are deposited while asmuch as 10 mg. of zirconium remain in solution. Another method is theprecipitation of the protoactinium with manganese dioxide. In thisprocedure a certain amount of zirconium is also carried along with themanga-- nese dioxide; this small amount of zirconium can be removed fromthe manganese by precipitation as zirconium phosphate which carries theprotoactinium with it. After dissolving the zirconium-protoactiniumphosphate in dilute hydrofluoric acid solution, the protoactinium can bere-.

moved by another manganese dioxide precipitation and this cycle may berepeated as often as is necessary in order to remove practically all thezirconium from the In a third method the protoactinium is dilutehydrochloric acid solution; six or eightprecipitations will carry aboutpercent of the protoactinium, while carrying a negligible amount of thezirconium, which remains in solution as a complex oxalate. (Theprotoactinium can then be removed from the lanthanum oxalate bydissolving the latter in hydrochloric acid and precipitating a verysmall amount of zirconium phosphate from this solution.) A fourth methodinvolves the fractional precipitation of zirconium iodate. If there isadded to a 1 to 6 N I-ICl or HNO solution, containing a mixture ofzirconium and protoactinium, a small amount of sodium iodate, sufficientto precipitate only a small fraction of the zirconium as zirconiumiodate, it is found that the protoactinium concentrates to a largeextent in this first fraction. This precipitate can be dissolved inconcentrated hydrochloric acid and again a partial precipitation ofzirconium iodate performed; in eachcycle a large concentration ofprotoactinium is obtained. By removing several zirconium iodateprecipitates in each cycle, the protoactinium can be brought alongquantitatively. Finally, a solution containing a salt of zirconium andPa in solution can be made 6 to 10 N in hydrochloric acid and subjectedto fractional crystallization for the separation of zirconiumoxychloride in crystalline form, whereupon the protoactiniumconcentrates in the mother liquor.

Example 2 A. solution containing 300 grams per liter of neutronirradiated thorium nitrate tetrahydrate and an acidity of 1 N nitricacid was prepared. This solution contained about 2 parts by weight of Pa|U per million parts of thorium. Manganous nitrate was added in excessof the amount required to form 1.1 grams per liter of Mn0 and sufficientpotassium permanganate was added to precipitate a total of 1.1 grams perliter of MnO in two approximately equal portions and the MnO containingthe Pa was recovered.

The MnO- was dissolved in a mixture of nitric acid and hydrogen peroxideand the solution was boiled to decompose excess peroxide and diluted toabout 20 grams per liter of manganous nitrate per liter and an acidityof 1 normal nitric acid. MnO was again precipitated in theproportionequivalent to about 10 percent by weight of the manganous nitrate insolution. The process of dissolving and precipitating Mn0 was repeatedseveral times and the MnO finally obtained was dissolved in nitric acidand concentrated to 4 M m-anganous ion. Thereupon the aqueous solutionwas permitted to age until a major portion of the Pa in solution haddecayed to U This U was then extracted from the solution with ether.

In accordance with a further modification of the invention, othermethods may be used for recovering a Pa concentrate which may be aged toform U For example an aqueous solution of neutron irradiated thoriumsuch as a nitric acid solution may be treated with hydrogen fluoridesubstantially in excess of that required to form thorium fluoride. Insuch a case the thorium precipitates leaving Pa in solution. This Pa maythen be treated to recover a Pa by convenient means such as we haveheretofore described.

Moreover the Pa may be recovered or removed as a concentrate from anitric acid acidified solution by precipitation of about 10-20 percentby weight of the thorium as thorium iodate which carries down the Pa inthe solution. For example a thorium nitrate solution having a nitricacid normality of about 4 may be treated with suflicient potassiumiodate to precipitate 10-20 percent of the thorium as thorium iodate inorder to remove the Pa. This precipitate may be redissolved andretreated in order to increase the Pa concentration if desired.

In the specification and claims, wherever reference is made to thepresence or the addition of phosphoric acid or a phosphate, unlessotherwise indicated by the context, it is to be understood that thereference is to orthophosphoric acid or its salts.

Bythe above methods of separating 1 21 and U from foreign products weare able to obtain compositions composed largely or entirely of Ucompounds, which are substantially free from fission products. The Umetal maybe produced from suitable compounds thereof by calciumreduction or any of the other known methods for producing uranium metalfrom compounds of uranium.

U metal or compounds of U may be shaped into the form of spheres,cylinders, blocks or the like by known methods of shaping uranium metaland cornpounds. Such shaped articles of manufacture may be used as asource of nuclear power as described in our copending application,Serial No. 565,990, filed November 30, 1944'.

While there has been described certain embodiments of'ou'r invention, itis to be understood that it is capable of many modifications. Changes,therefore, may be made without departing from the spirit and scope ofthe invention as described in the appended claims, in which it is theintention to claim all novelty in the invention as broadly as possible.

We claim:

1. The method of separating Pa from foreign products present in neutronirradiated thorium which comprises forming a solution of neutronirradiated thorium and a manganous salt, then adding a substanceselected from the class consisting of soluble manganates andpermanganates to precipitate manganese as manganese dioxide wherebyprotoactiniumis carried down with the manganese dioxide.

2. The method of separating Pa from foreign products present in neutronirradiated thorium which comprises forming a solution of neutronirradiated thoriumand a manganous salt, then adding potassiumpermanganate to precipitate the manganese as manganese dioxide wherebyprotoactinium is carried down with the manganese dioxide; dissolving theprecipitate, adding a soluble zirconium salt, and adding phosphate ionto precipitate zirconium phosphate whereby protoactinium is carried downwith the zirconium phosphate.

3. The method as in claim 1, wherein potassium permanganate is employedas the substance for precipitating manganese dioxide.

4. A method of recovering Pa which comprises forming a solution ofneutron irradiated thorium containing Pa and contacting the solutionwith manganese dioxide which removes Pa from the solution.

5. The method of separating protactinium from uranium and thorium, whichcomprises forming a solution containing ions of said elements and amanganous salt, then adding a substance selected from the classconsisting of soluble manganates and permanganates to precipitatemanganese as manganese dioxide whereby protoactinium values are carrieddown with the manganese dioxide.

6. The method of separating protoactinium from uranium and thorium,which comprises forming a solution containing ions of said elements anda manganous salt, then adding potassium permanganate to p1ecipitate themanganese as manganese dioxide whereby protoactinium is carried downwith the manganese dioxide, dissolving the precipitate, adding a solublezirconium salt, and adding phosphate ion to precipitate zirconiumphosphate whereby protoactinium is carried down with the zirconiumphosphate.

7. A method of separating Pa from zirconium which com-prisesprecipitating Pa with a carrier precipitate of manganese dioxide from anaqueous solution containing zirconium and Pa 8. method of separatingpro'toactinium values from zirconium which comprises precipitatingprotoactinium values with a carrier precipitate of manganese dioxidefrom an aqueous'solution containing zirconium and protoactinium.

References Cited in the file of this patent UNITED STATES PATENTSSchwerin Dec. 8, 1914 Fermi et a1 July 2, 1940 OTHER REFERENCES

1. THE METHOD OF SEPARATING PA233 FROM FOREIGN PRODUCTS PRESENT INNEUTRON IRRADIATED THORIUM WHICH COMPRISES FORMING A SOLUTION OF NEUTRONIRRADIATED THORIUM AND A MANGANOUS SALT, THEN ADDING A SUBSTANCESELECTED FROM THE CLASS CONSISTING OF SOLUBLE MANGANATES ANDPERMANGANATES TO PRECIPITATE MANGANESE AS MANGANESE DIOXIDE WHEREBYPROTOACTINIUM IS CARRIED DOWN WITH THE MANGANESE DIOXIDE.